Energy shortage is a critical issue to be solved in the future. Currently, the major countries in the world are seeking actively renewable energy sources such as solar energy or wind energy to replace traditional petrochemical fuels. In particular, thanks to its advantages of inexhaustible supply and safety, because no waste is produced during the power generation, the solar technology has become the key industry to which the major industrial countries worldwide have invested massive researches.
Among solar cell technologies, due to their advantages of simple fabrication processes, low cost, perviousness to light, and integrable with flexible substrates, organic solar cells are valued increasingly.
In order to reduce the cost of organic solar cells, there is a technology presently for fabricating large-area organic solar cells. The technology abandons the technique of spin coating for preparing solar cell thin films. Instead, large-area processes, such as spray coating or roll-to-roll (R2R) coating, are adopted. Currently, for preparing organic solar cells, the wet process is mostly adopted, in which the material for the active layer is first dissolved in a solvent for forming an active layer solution. Then spin coating or roll-to-roll coating is adopted to form the active layer. In recent years, the efficiency of organic solar cells increases substantially. This is attributed to the maturity of the novel synthesis technology for low-bandgap polymers. Nonetheless, in the system using low-bandgap polymers as the active layer, in order to enable the devices in the system to have superior power conversion efficiency, different additives with high boiling point, such as DIO, CN, or NMP, are normally added to the active layer solution. By doing so, the active layer can have better internal nanometer structures, electron donor and acceptor materials are separated appropriately, and the conductive high-polymer crystals are superior and hence facilitating absorption of photons, separation of excitons, and conduction and collection of charges. Consequently, superior power conversion efficiency is achieved.
The method for fabricating organic solar cells according to the prior art is spin coating. This method can dry the wet film in a short time. In the system having high-boiling-point additives, due to rapid film forming, the residual high-boiling-point additives in the dry film differ. These residual high-boiling-point additives affect the stability of the efficiency of organic solar cells. That is to say, the production yield is inferior. In addition, residual additives also affect significantly the lifetime of devices. Accordingly, how to remove the additives has become an emphasis of process improvement.
The methods for removing residual additives according to the prior art include spin coating an extra layer of alcohol solvent. By taking advantage of high solubility of the additives in alcohol, the residual additives can be removed. Alternatively, long-term heating at temperatures (80˜100° C.) higher than room temperature facilitates vaporization of the additives from films. Besides, there is also a technique of drawing out the additives in a high vacuum method. These methods require either long-term process or additional processes to reduce the residual additives. These impose a serious limitation on commercial mass production.
On the other hand, in the roll-to-roll mass-production processes, owing to much slower rate of film forming compared with spin coating, the residual additives in active layers are much greater than those in the traditional spin coating process. In addition, because the roll-to-roll process is a continuous process, films must be dried before they are rolled, otherwise they will be damaged. Thereby, the films should pass through an oven set at a fixed temperature. Nonetheless, the influence of the baking temperature on residual additives for the current fixed-temperature drying technique on roll-to-roll organic solar cells using an oven is still unclear; the influence of the drying temperature of films on efficiency and yield is unknown as well. Consequently, the yields for different batches of products vary and the stability of product quality is affected.
Accordingly, an appropriate roll-to-roll mass-production process can control the residual additives and improve the overall large-area production yield. In order to realize commercialization of organic solar cells in the future, an appropriate roll-to-roll mass-production process plays a key role.